It is well known that in order to remedy that drawback, the arcing voltage must be increased by using suitable means. A high arcing voltage makes it possible to absorb the energy from the DC component of the current and to cause the DC component tend towards zero.
Using an interrupting chamber having permanent fuses in series with a conventional high-voltage circuit-breaker has been proposed. During breaking as a result of a fault, the fact that the fuses melt produces a very high arcing voltage which causes the DC component of the fault current to decrease very quickly.
Using a high-pressure interrupting chamber equipped with means for creating a plurality of arcs in series has also been proposed.
Such solutions require new equipment to be used. An object of the present invention is to make a circuit-breaker that is capable of creating a high arcing voltage, by using existing equipemnt that is available on the market, and making only a cheap minor technical modification.
It is known that high-pressure compressed-air circuit-breakers can create high arcing voltages that are more than ten times the arcing voltages of sulfur hexafluoride (SF.sub.6) circuit-breakers. Such conventional compressed-air circuit-breakers have been used for a long time now in high-voltage and very high-voltage grids. To this end, reference is made to the article "Appareillage haute tension" by E. Thuries et Pham Van Doan, Avenir du Genie Electrique, Colloque National organized by the Ministere de la Recherche, Actes, 28-29 January 1987, Paris.
Compressed-air circuit-breakers are used in particular in very cold regions where ambient temperature can drop to -50.degree. C.
At that temperature, the SF.sub.6 may condense if the pressure inside the interrupting chamber of the circuit-breaker is not low enough.
U.S. Pat. No. 4 204 101 describes a vacuum interrupter and an SF.sub.6 interrupter associated together in series. That association does not enable the arcing voltage to be increased.
U.S. Pat. No. 3 227 924 describes two circuit-breakers associated together in series having different dielectric recovery rates. That association makes it possible to maintain electrical isolation after interruption, but not to increase the arcing voltage before interruption.
The invention provides a hybrid circuit-breaker including, for each phase, compressed-air interrupting chambers and SF.sub.6 interrupting chambers, the resulting assembly making it possible to interrupt fault currents that pass through current zero with a delay because of their high DC component.
The purpose of compressed-air interrupting chambers is similar to that of permanent-fuse interrupting chambers or to that of multiple-arc interrupting chambers: it is to create an arcing voltage that is sufficiently high.
The purpose of the SF.sub.6 chambers is to interrupt the current on current zero and to isolate the circuit.
A resistor of several thousand ohms or a capacitor of the order of several hundreds of thousands of picofarads is disposed across the terminals of each compressed-air interrupting chamber. As a result, when in the open position while live, there is no voltage stress on the compressed-air interrupting chambers. It should be noted that at the time of interruption, the arcing voltage may reach several tens of kilovolts. Using a high-capacitance capacitor of several hundreds of thousands of picofarads across the arcing terminals may make the arc unstable because of current oscillations via a low-inductance inductor or via the inductance of the connection in series with the capacitor. Such instability helps to increase the arcing voltage and to decrease the current towards zero. To this end, reference is made to the following articles:
"L'oscillation arc-capacite et son utilisation pour l'evaluation des parametres caracteristiques d'un arc electrique" by A. Mohssen Zeneldien, Thesis, University of Grenoble, 5 June 1974;
"HVDC circuit breakers using oscillating current techniques", by Yamada et al., Direct Current, 1966, No. 8; and
"DC circuit breaker", U.S. Pat. No. 4 216 513 of 5.ANG.August 1980, Hitachi Ltd.
The absence of voltage stress on the compressed-air interrupting chambers means that the casings for the compressed-air interrupting chambers can be made of a composite material, e.g. glass fibers impregnated with epoxy, with fins made of silicone or of EPDM. High compressed-air pressures can then be used and any explosions that are always possible when ceramics are used can be avoided.
In order to provide near-perfect operating safety, the compressed-air interrupting chamber re-closes automatically, i.e. after it has been opened, its moving contact re-closes automatically and quickly in a few hundredths of a second instead of several tens of hundredths of a second, as is usually the case in conventional circuit-breakers in which electrical open or close commands are given on the basis of the indications supplied by the auxiliary contacts (also referred to as "signal contacts").